In conventional marking systems, such as, for example, a laser printer, inkjet printer, or copier, one technique for monitoring the quality of images is to create one or more “reference” or “test” patches of pre-determined desired tint. The reference/tint of a test patch may be referred to as the call or the density of the reference/test patch. The actual density of the material (often ink and/or toner) in each test patch can then be measured to determine the effectiveness of the printing process on marking a medium, such as for example, a reference strip on a photoreceptor or intermediate medium. The uniformity of the image on the reference strip can then be determined.
It is known that non-uniformity in the appearance of printed materials intended to be uniform is a persistent problem for marking technologies, such as direct-digital production color technologies. Thus, marking machines have inherent error manifesting itself in residual non-uniformities, even after all normal service actions, such as machine self-check diagnostics and technician implemented procedures, have been performed on a marking machine. These residual non-uniformities may occur, for instance, where an image to be printed is intended to be a specific uniform tone, but shows areas which are lighter or darker, or a different tone than other areas. These different areas of the same image are variations that were not intended when the image data was generated and do not reflect the image data generated. The ability to assess and diagnose unwanted non-uniformity is a problem for field service personnel. Engineering tools such as densitometers, two-dimensional precision color scanners, digital cameras, flat bed cameras, and elaborate signal processing which may be available in the lab are generally unavailable to field service personnel who must use simpler and less capable tools. Generally, field personnel must use printed standard image references (SIR) and visual comparisons to determine whether a printing system meets its specified uniformity performance. Additional transparent overlays are placed on printed images to determine spatial frequencies of unwanted image bands. The processes are subjective and thus, have a tendency to be inaccurate.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent. Test patches are used to measure the deposition of toner on paper to measure and control the tone reproduction curve.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a light sensing device disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch. The sensor readings are then used to make suitable adjustments to the system such as changing developer bias to maintain consistent quality.
Typically each patch is about an inch in the process direction and extending the full width across the process that is printed as a uniform solid half tone or background area. This practice enables the sensor to read one value on the tone reproduction curve for each test patch.
Often, however, when a full-width array sensor module that uses a lens array for imaging captures images in specular reflecting mode, spatially localized non-uniformities of the sensor response are seen. When these same types of sensors are used in a diffuse mode they also exhibit non-uniformities but they are not the same size or in the same locations along the array width. Normal calibration techniques of dark offset and gain correction eliminate almost all effects of these non-uniformity errors when the sensor is used to measure the reflectance of inks or toners that are very light-absorbing.
However, if the material being imaged has a significant high diffuse reflectivity and the sensor is being used in a predominantly specular reflecting mode, then the difference of the two non-uniformities shows up uncorrected in the images. This may result in artifacts such as narrow point streaks during the image processing and subsequent bitmap manipulation.